LibCore: Put all classes in the Core namespace and remove the leading C

I've been wanting to do this for a long time. It's time we start being
consistent about how this stuff works.

The new convention is:

- "LibFoo" is a userspace library that provides the "Foo" namespace.

That's it :^) This was pretty tedious to convert and I didn't even
start on LibGUI yet. But it's coming up next.

Libraries/LibGUI/GRadioButton.h

@@ -53,7 +53,7 @@ private:
 };
 
 template<>
-inline bool is<GRadioButton>(const CObject& object)
+inline bool Core::is<GRadioButton>(const Core::Object& object)
 {
     if (!is<GWidget>(object))
         return false;